This invention relates to metal deforming, and more particularly, to deforming a flat metal strip into a round tube by sequentially passing such strip between spaced pairs of shaped rollers.
Mills are well known in the prior art for producing welded metal tubes. The mills typically include a series of pairs of driven and idler rollers which, while advancing an elongated flat metal strip, impart an increasing curvature to such strip to form a tube. After forming, the tube passes through a welder which heats the edges of the strip and forges such edges together to form a continous welded tube. The welded tube is then cooled, passed through a sizer to impart a final round shape and then cut into preselected lengths. In the past, the idler rollers in each pair, which initially shape the flat metal strip into a curved tube, have been mounted on two spaced parallel shafts. The two shafts are adjustable towards and away from one another and may be raised and lowered to position the idler rollers for alignment to receive the advancing metal strip therebetween Each idler roller in a pair imparts one half of a predetermined symmetrical curve to the metal strip. The idler roller includes a lower surface area which extends to substantially the center of the advancing metal strip and an upper surface area which contacts an edge of the advancing metal strip. Since the rollers rotate about parallel vertical shafts, the lower surface area of each roller has an appreciably larger diameter from the center of rotation than the upper surface area. As a consequence, the surface speed of the smaller diameter upper surface area of the roller contacting the edge of the metal strip is appreciably lower than the surface speed of the larger diameter lower surface area of the roller which contacts the center of the strip. This speed differential is highly undesirable since it causes a scouring effect adjacent the center of the strip which in turn produces scuff marks on the finished tube and, also, it causes a distortion in the form of edge stretch on the metal strip. The high surface speed differential between different portions of the roller and the metal strip results in considerable energy loss. Thus, a higher powered motor is required to advance the metal strip between the pairs of idler rollers than is required merely for the shaping operation. In addition, the surface speed differential causes considerable wear on the rollers which shortens the life of the rollers. Since the lower surface area of the roller is of a relatively large diameter, the cost of manufacturing the roller is high. Therefore, it is desirable to design a roll former for use in a tube mill which is more efficient over prior art roll formers.